STEP7: Event ID 16# 4575

HI, the problem is with PLC error, aftet part of time working the PLC
stops with message (diagnostic buffer) :

Event 1 of 100:  Event ID 16# 4575
STOP caused by exceeding block stack nesting depth in the priority class
stack
Caused by OB:    0 Priority class:    0
Previous operating mode: Configuration in RUN
  Requested operating mode: STOP (update)
Internal error, Incoming event
10:21:56:480 am  04/10/06

STEP's hint: reduce the number of nested block calls
The program is simply and has only one call function. where could be the
problem ?